A cylinder pressure sensor is disclosed, for example, in Unexamined Published Japanese Patent Application No. 7-306109 (hereinafter referred to as "conventional art 1"), wherein an optical fiber is subjected to bending depending on a variation in cylinder pressure, so that the cylinder pressure is detected by the variation in the amount of light being transmitted through the optical fiber, thereby enhancing the measurement accuracy.
Meanwhile, Unexamined Published Japanese Patent Application No. 8-122188 (hereinafter referred to as "conventional art 2") discloses a sensor that is simplified as compared to the structure of the conventional art 1, to thereby achieve a reduction in cost.
The cylinder pressure sensors of the conventional arts 1 and 2 are effective in terms of enhancing the accuracy in measuring the cylinder pressure. However, they have a limitation as to the frequency that can be measured. There is, therefore, a problem in that it is impossible to accurately detect vibrations occurring in a high frequency range (hereinafter referred to as "knocking") at a pressure of a low as approximately 1/1000 of the cylinder pressure.
On the other hand, a cylinder pressure sensor requires stable detection of signals over a long period of time, since it is mounted on an automotive vehicle. Furthermore, it is preferred for the cylinder pressure sensor to structurally avoid initial variations in the cylinder pressure and the knocking signals for each cylinder.